The present invention relates to an apparatus and a method for controlling fuel injection in an internal combustion engine that includes a first fuel injection valve for injecting fuel into a cylinder and a second fuel injection valve for injecting fuel into an intake passage.
Conventionally, in an internal combustion engine having an in-cylinder injection valve for injecting fuel into a cylinder, “compression stroke injection” is performed to inject fuel into a combustion chamber during the compression stroke of a piston, so that stratified lean combustion, in which the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, is performed. In the stratified lean combustion, a combustible air-fuel mixture having the stoichiometric or richer air-fuel ratio is generated only in the vicinity of the ignition plug. Thus, even if the overall air-fuel ratio in the combustion chamber is lean, the combustion is stabilized. As a result, the fuel economy is significantly improved.
However, if, for example, the fuel injection amount falls below a requested fuel injection amount due to deposits collected on the nozzle of the injection valve, the air-fuel ratio of the air-fuel mixture in the vicinity of the ignition plug will be leaner than the stoichiometric air-fuel ratio, which can cause a misfire to occur. Such a misfire is likely to occur in an operational range of the engine in which the requested fuel injection amount is small, for example, when the engine is idling.
Japanese Laid-Open Patent Publication No. 2002-130007 proposes that stratified stoichiometric combustion be performed as a measure against misfires during the stratified lean combustion. In the stratified stoichiometric combustion, fuel is injected both during the intake stroke and the compression stroke so that the air-fuel ratio in the entire combustion chamber becomes the stoichiometric air-fuel ratio, thereby generating an air-fuel mixture of which the air-fuel ratio is richer than the stoichiometric air-fuel ratio in the vicinity of the ignition plug. As a result, misfires due to a lean air-fuel ratio are prevented.
Incidentally, in an internal combustion engine having an in-cylinder injection valve, a misfire can occur even during homogeneous stoichiometric combustion in which fuel is injected during the intake stroke. This is because when fuel is injected during the intake stroke, the injected fuel is not sufficiently diffused throughout the entire combustion chamber by the time of ignition. As a result, due to an inhomogeneous air-fuel mixture, the air-fuel ratio in the vicinity of the ignition plug is lean, which may cause a misfire to occur.
In this manner, as a measure against misfires caused by a lean air-fuel ratio in the homogeneous combustion, it is effective to perform the above described stratified stoichiometric combustion for richening the air-fuel ratio in the vicinity of the ignition plug.
As a measure against misfires due to a lean air-fuel ratio, increasing the fuel injection amount for richening the air-fuel ratio in the vicinity of the ignition plug is effective. However, performing the stratified stoichiometric combustion, in which the fuel injection amount is greater than that in the stratified lean combustion, lowers the fuel economy.
In the stratified stoichiometric combustion, an air-fuel mixture of which the air-fuel ratio is richer than the stoichiometric air-fuel ratio, is generated in the vicinity of the ignition plug as described above, so that the air-fuel ratio in the entire combustion chamber becomes the stoichiometric air-fuel ratio. Therefore, some of the fuel injected into the combustion chamber can be discharged without being combusted.